When operating wireless networks, problems can occur when multiple networks or multiple devices are operating using the same channel or the same bandwidth. Such problems can include contention problems leading to collisions, crosstalk, interference or the like. Furthermore, when operating within a regulated operating band, strict power levels must be observed to reduce interference while preserving performance and efficiency. Since some devices configured to work within the UWB spectrum are battery operated power efficiency is also a concern as is bandwidth utilization, channel efficiency, and the like.
One common approach to addressing the multiple device issue in wireless networking is the use of a frequency division multiple access (FDMA) scheme. Such an implementation is often used in narrow band systems. In an FDMA scheme, different networks or devices are assigned a different frequency band. In theory, provided each network or device uses its assigned portion of the spectrum, it can be assured that other networks or devices will not interfere within the assigned frequency portion. An example of FDMA is a conventional analog cellular radio or radiotelephone system with each voice channel being assigned a unique uplink frequency channel and downlink frequency channel within a cell or neighborhood of cells.
It will be appreciated that FDMA approaches are not readily applicable in a UWB environment since UWB transmissions by definition occupy a wide frequency band. In UWB systems, due to the use of narrow pulses in the time domain, transmission energy is spread across the frequency spectrum. Thus, for UWB systems it is not feasible to allocate mutually exclusive frequency bands within the available spectrum.
Another approach to contention in is to use a code division multiple access (CDMA) scheme. In a CDMA scheme, networks and devices transmit over the same frequency spectrum and at the same time, but signals from each are encoded using codes specially chosen to minimize their interference with each other by maximizing isolating characteristics of the transmissions. Characteristics include code distance or code separation associated with codes used in transmissions destined to one receiver from those destined to another receiver.
CDMA schemes also have limitations. First, the code set having the desired isolation properties needed to keep overlapping networks and devices separate is finite limiting the number of networks or devices that can operate at the same time in a given area. Second, code separation provided by the use of CDMA codes does not result in perfect isolation particularly given effects of multipath, fading, and the like. As a result a near-far problem may arise where a close device of a different network can overpower and obliterate a signal from a distant device of the same network despite the use of codes by the close device chosen to minimize interference with the other network.
Yet another approach for handling multiple devices is the use of a time division multiple access (TDMA) scheme. In a TDMA scheme, the available transmission time is allocated into multiple time slots, and each network or device is assigned one or more of the time slots. Thus, each device is given some portion of the available transmission time to use and remains silent during all other times. In a TDMA scheme each network or device experiences an overall reduction in transfer speed, particularly for large quantities of data, since transmission is limited to dedicated portions of the total available channel time. As the number of overlapping networks or devices increases, transfer speed for individual devices is correspondingly reduced. For example, a 100 megabits per second (Mbps) capacity divided evenly over four separate networks or devices results in a 25 Mbps transmission speed for each individual device or network. It will be appreciated that other factors such as the likelihood of error combined with the need for acknowledgments (ACKs), the need to retransmit erroneous or dropped packets on a particular channel, and the packet retransmission scheme, can also affect the net transfer speed for each device or network.
One way to transmit more data in a TDMA scheme is to increase the transmission power for a given network or device. In a digital system, for example, using a stronger signal means that each individual bit of data requires less time to send, enabling the device to operate at an increased data rate. A significantly increased transmit power can, therefore, compensate for time lost when other networks or devices are transmitting. However since, in the United States, the Federal Communications Commission (FCC) imposes restrictions such as limits on the maximum allowable transmit power for UWB signals, and since the likelihood is strong of similar agencies in other countries imposing similar restrictions, the maximum capacity for UWB systems is limited.
Therefore, if a TDMA scheme is used in the UWB signal transmission environment, it is desirable to maximize the transmit power while minimizing duration in the time domain without violating the maximum signal power restrictions set up by the FCC or similar regulating agency. It is also desirable to achieve greater efficiency in multi-user environments while supporting a number of transmission scenarios, synchronous, asynchronous, isochronous, and the like, and various media access control (MAC) configurations, such as TDMA, carrier sense multiple access (CSMA), polling, and the like.